Extrusion coating process for catalytic monoliths

ABSTRACT

A high throughput, automated process for coating ceramic monoliths used as catalytic converters that overcomes problems with coating monoliths individually in a mold. The monoliths are provided with end caps to cover the open functional ends, then loaded serially into an inlet channel leading to an extrusion chamber. As each monolith is pushed into the channel, one monolith enters the extrusion chamber, and the monolith just coated in the extrusion chamber is made to exit onto a finished part conveyor. Thereafter, the end caps are removed and cleaned for reuse on new monoliths yet to be coated.

BACKGROUND AND SUMMARY OF THE INVENTION

[0001] This invention relates to a method of applying a seal to catalystmonolith structures used in catalytic converters. More particularly itrelates to an apparatus and a method for the extrusion of a seal coatingonto the outer surface of such structures.

[0002] Catalytic converters are used for catalytically treating theexhaust gas of internal combustion engines. A conventional catalyticconverter includes a relatively fragile ceramic monolithic catalyststructure which has a catalyst such as platinum deposited thereon. Thecatalyst structure typically has a cylindrical side surface and openfunctional end surfaces. Exhaust gases are treated by passing themthrough the catalyst structure from the input end to the output end. Ina catalytic converter the catalyst structure is mounted within a metalhousing.

[0003] A seal is disposed between the metal housing and the catalyststructure to support the monolithic catalyst structure within thehousing. The seal fills the space between the housing and the catalyststructure so that gases can not leak through this space and so thatsubstantially all of the exhaust gases entering the inlet end of thecatalytic converter will pass through the catalyst structure. Inaddition to preventing gases from bypassing the catalyst structure, theseal holds the catalyst structure in place within the housing, therebycushioning this relatively fragile ceramic structure to prevent breakageresulting from physical shocks due to mechanical vibration typical ofautomotive vehicles.

[0004] The seals used in catalytic converters are often made of a pasteis coated onto the exterior lateral surface of the catalyst structure.This seal material must be able to withstand temperatures in excess of2000° F. since the catalytic converter will be exposed to suchtemperatures during operation. It must also be capable of holding thecatalyst structure in place within the housing while dampeningmechanical vibration between the housing and the catalyst structure.

[0005] Such a temperature resistant seal may, for instance, be made froma mixture in which an aqueous dispersion of high aspect ratiovermiculite is blended with ceramic fibers to produce a relativelysmooth formable blend of vermiculite and fibers. A paste formed of thismixture is applied to the exterior lateral surface of the monolithiccatalyst structure so that it encircles the structure while leaving thefunctional end surfaces uncovered so that exhaust gas can pass through.

[0006] Examples of mixtures suitable for such seals are disclosed inU.S. Pat. Nos. 5,207,989 and 5,385,873 and in British Patent No.1,522,646, the disclosures of which are incorporated herein byreference. One example of such a material is a mixture comprisingbetween approximately 28 and 80 dry weight percent of ceramic fiberscapable of withstanding continuous exposure to temperatures in excess of2,000° F., between approximately 20 and 50 dry weight percent of anaqueous dispersion of high aspect ratio vermiculite and preferably alsobetween approximately 5 and 60 percent dry weight percent of lowtemperature expanded vermiculite.

[0007] In the prior art, the seal material is applied to the catalyststructures by placing the catalyst structure in a mold and injecting theseal material between the mold and the catalyst structure to coat thestructure. The coated catalyst structure is then heated to removeliquids from the seal material, effectively curing it in place aroundthe monolithic catalyst structure. This process is time consuming sinceeach catalyst structure must be separately coated with the sealmaterial. In addition, there is a danger of inadvertently applying theseal material to the functional end surfaces of the catalyst structure.Alternatively, the catalyst structure could be mounted within thehousing of the catalytic converter and the seal material could beinjected into the space between the catalytic structure and thesurrounding housing. Injecting the material directly into the spacebetween the housing and the catalyst structure can result in damage tothe relatively fragile ceramic structure and/or can fail to provide auniform seal between the structure and the housing, since the sealcannot be inspected as it may be in the molding operation.

[0008] It is an object of the present invention to coat monolithiccatalyst structures with seal material by a high speed extrusion coatingoperation in which a number of monoliths are passed through an extrusionchamber in which they are coated with the seal mixture in a continuousoperation. This continuous process methodology shortens the time neededto apply the seal coating to each of the monoliths, thereby resulting infaster production and a cost savings.

[0009] It is a further object of this invention to apply a uniform andsmooth seal coating to the lateral sides of monolithic catalyststructures, especially without having to coat each monolith separatelyin a mold.

[0010] Another object of this invention is to apply a seal coating tothe lateral sides of the monolithic catalyst structures in a manner thatwill protect the functional end faces of the catalyst structures frombeing contaminated by the seal material.

[0011] Other objects, features and advantages of the invention willbecome apparent from the following description, when considered inconnection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In the drawing which illustrates the best mode presentlycontemplated for carrying out the present invention:

[0013]FIG. 1 is a schematic illustration of the apparatus used in theinvention to continuously coat monolithic catalyst structures.

DESCRIPTION OF THE INVENTION

[0014] Referring to FIG. 1, a number of monolithic catalyst structures10 are shown on a loading conveyor 12. End caps 14 a, 14 b, which may bemade of rubber or other suitable material, are placed over eachfunctional end of the catalyst structures. The catalyst structures withtheir end caps are transported by the loading conveyor 12 to the entry16 of an extrusion chamber 18.

[0015] A conventional and commercially available pick and placeapparatus 22, which may be a robot arm, positions each catalyststructure so that it is aligned end to end with other such catalyststructures to form a column of such structures at the entry to theextrusion chamber 18. A linear pushing device 24 then engages the endcap of the last catalyst structure in the column and pushes on thecolumn to move one of the catalyst structures into the extrusion chamber18.

[0016] The end caps 14 a, 14 b separate the individual catalyststructures in the column from each other and space the structures sothat the structures will be properly positioned in the extrusionchamber. The end caps 14 a, 14 b also cover the functional end faces ofeach catalyst structure to prevent the application of seal material tothese surfaces as the structures pass through the extrusion chamber.Still further, the end caps form seals above and below the monolith tomaintain the seal material within the extrusion chamber.

[0017] The pusher mechanism 24 is synchronized with extruder pumps 26 a,26 b. As the pusher mechanism 24 moves the column of catalyticstructures so that one of the structures enters the extrusion chamber18, the extruder pumps supply the seal material to the extrusion chamberthough feed lines 28 a, 28 b. Vacuum vents 30 a, 30 b remove air from(de-air) the extrusion chamber 20 to ensure that entrapped air does notprevent the available space around the catalyst structure from beingcompletely filled with the sealing material, and allowing the sealmaterial to be applied over the entire lateral surface of each of thecatalyst structures. De-airing is continued for a time effective toremove air bubbles entrapped in the seal material. The extruder pumpsare preferably reciprocating piston pumps, driven by servo motors, andthe amount of seal material pumped is controlled as a function of themonolith speed through the extrusion chamber.

[0018] When the pusher mechanism moves the next catalyst structure intothe extrusion chamber to be coated, it forces the preceding coatedcatalyst structure in the column out of the chamber. The cycle time ofthe pusher mechanism is chosen so that each of the catalyst structureswill be in the extrusion chamber for a sufficient period to ensure thatits lateral surface is completely coated with the seal material. Thecycle time can be as short as two seconds, or even shorter, depending onthe size of the monolith, the dimensions of the chamber, the vacuumvents, and the size of the manifolds or feed lines with respect to thehead developed by the extruder pumps.

[0019] As it passes out of the extrusion chamber 18, the catalyststructure passes through a calibrating ring 30 which strips away excessseal material from the structure's lateral surface and provides a smoothseal surface coating of the desired thickness. The coated monolith exitsthe extrusion chamber onto a finished parts conveyor 32. The rubber endcaps 14 a, 14 b are then removed from each of the catalyst structureseither manually or, more preferably, by suitable robotic equipment.These end caps can then be cleaned and recycled for use on othermonoliths yet to be coated. The excess material, while preferablyremoved by a calibrating ring at the outlet of the extrusion chamber,can be removed in a separate step after the coated structure has beenremoved to the finished parts conveyor. For example, a ring-like cuttercan be made to come down over the coated structure when it resides at aspecific location, similarly stripping away excess seal material; or arotating table may be interposed in the coveyor, and the coated partrotated adjacent to a stripping or doctor blade.

[0020] The pusher mechanism, extruder pumps, and the final part conveyorare all synchronized by a programmable logic controller.

[0021] After they are coated with the seal material, the catalyststructures are then placed in a drying oven. The drying oven heats themonolith and its seal coating so that substantially all of the waterevaporates from the seal material. After drying, the finished coatedcatalyst structure is either stored for future use or is mounted in ahousing to complete the assembly of a catalytic converter.

[0022] The apparatus described above can be used to coat catalyticmonoliths having different diameters and shapes by utilizing anextrusion chamber of an appropriate size and shape. The shape of theextrusion chamber can be changed by removing and replacing the chamberwith a more appropriate device, or the extrusion chamber can be made tohave variable walls, either solid that are fixed by pneumatics,hydraulics, or mechanics, or may have pliable walls that are backed by apneumatic, hydraulic, or mechanical device to change their shape. Thismolding apparatus provides a continuous, high volume production flow ofcoated monoliths with minimum maintenance. The use of an extrusionchamber to coat the monoliths results in close tolerances on thediameter of the seal coating and a smooth seal surface.

[0023] While specific apparatus has been shown and described whichembodies the invention, it will be apparent to those skilled in the artthat various modifications and rearrangements of the parts may be madewithout departing from the spirit and scope of the inventive concept.Therefore the invention is not meant to be limited to the specificembodiments described and shown herein except as indicated by the scopeof the appended claims.

What is claimed is:
 1. Apparatus for applying a seal material to thelateral surfaces of monolithic catalyst structures having a lateralsurface and two end surfaces comprising: an extrusion chamber; an inletto the extrusion chamber adapted to receive a series of catalyststructures in end-to-end alignment forming a column of such structures;a pusher acting on the last structure in the column to move one of theseries of catalyst structures into the extrusion chamber and to move thepreceding catalyst structure in the column out of the extrusion chamber;an evacuation apparatus effective for removing enough air from theextrusion chamber to ensure that the area of the chamber around acatalyst structure disposed therein will be filed with seal material; aseal material delivery apparatus including extrusion pumps adapted tosupply seal material to the extrusion chamber at a predetermined timewhen a catalyst structure is disposed therein, and means for removingexcess seal material from the coated catalyst structure.
 2. Apparatus asclaimed in claim 1, further comprising means for adding and/or removingend caps over the end surfaces of each of the catalyst structures toprotect the end surfaces from the seal material.
 3. Apparatus as claimedin claim 1, further comprising a drying oven to remove liquid from theseal material coated onto the catalyst structures.
 4. A method forcontinuously applying seal material to monolithic catalyst structures,each structure having a lateral surface and two end surfaces,comprising: applying temporary sealing the end surfaces of each of thecatalytic structures; aligning the catalyst structures end to end toform a column; moving the column of catalyst structures a discretedistance so that one of the catalyst structures moves into an extrusionchamber; removing enough air from the extrusion chamber to ensure thatthe available space within the chamber will be filled with sealmaterial; supplying seal material to the extrusion chamber effective tocoat the structure; moving the column of catalyst structures a discretedistance effective to push another structure into the chamber and toremove the structure just coated from the chamber; and removing thetemporary sealing from the end surfaces of the coated structure.
 5. Amethod as claimed in claim 4, further comprising removing excess sealmaterial from the coated structure.
 6. A method as claimed in claim 4,whererin the temporary sealing comprises end caps.
 7. A method asclaimed in claim 6, including the additional steps of cleaning andrecycling the end caps for use on other structures to be coated.
 8. Amethod as claimed in claim 4, including the additional step of removingwater from of the seal material coated onto the structure.
 9. A methodas claimed in claim 8, wherein the water is removed by heating in adryer.
 10. A method as claimed in claim 4, wherein the steps of movingthe column, removing air, and supplying seal material are synchronizedusing a control system.